Near field or RFID tags are known. Such tags may be incorporated into smart cards or other devices for use in obtaining entry to buildings or as electronic tickets for use of public transport or toll fee payment. Although such tags typically have to be brought into close proximity (less than 20 centimeters) of a reader, it is possible for a reader to exchange information with the tag without the owner's permission. Sensitive data stored on the tag could be stolen or changed. The tag may therefore participate in a sensitive or valuable transaction without the knowledge of the owner.
Known devices incorporating such tags may be provided with some protection against such mis-use. For example, the RF link between the card and the reader is of limited range, as discussed above. This should mean that the holder of the device incorporating the tag will be able to see the reader. However, it is possible for specialist radio equipment to extend the intended range within which communication between the tag and the reader can occur, and thereby exchange information with the tag without the owner's permission.
It is also known to encrypt the communication between the tag and the reader. Although encryption can be effective, it does not guard against unauthorised communication with a rogue reader, such as a genuine reader that has been stolen or a reader that has broken the encryption algorithm.
Another known but not widely used security precaution is to place the device including the tag in a metal or other enclosure that provides shielding to prevent RF access to the card. Although shielding is effective, it is not user convenient as it requires the user to remember to place the card in the shield. Further, if the user has several devices incorporating such tags, this can become even more inconvenient.